Is the Catholic teaching/interpretation on the unforgivable sin considered infallible?
I don’t know what your understanding of the unforgivable sin is.
I believe the unforgivable sin is when someone thinks that God cannot forgive him.
Perhaps a better way of saying it is that final impenitence is the unforgivable sin. (When a person obstinately does not want to ask forgiveness of God.)
Here’s St. Thomas Aquinas on the topic. Lengthy, but a good read.
I was mainly curious if the portion of the catechism that talks about the unforgivable sin is considered infallible?
If you have a copy of the Catechism of the Catholic Church, it generally is published with an introduction by Pope St. John Paul II who described the catechism as the “deposit of faith.”
It is not intended to be a commentary on the Bible, as if that were the deposit of faith (or not). The Bible and the Catechism stand independently but are closely related. The Catechism is an explanation of a*** lot ***of the Bible, but not exhaustively.